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LMX2571: Monotonic frequency changes

Rob Gaddi
Prodigy 130 points

Part Number: LMX2571
Other Parts Discussed in Thread: LMX2820

I'm using the LMX2571 as a frequency synthesizer, and I'd like to have the output of the LMX2571 transition (mostly) monotonically from the currently programmed frequency to a new one.  As quickly as possible, but that's a secondary concern to not having the frequency go berserk in between two closely-related frequencies.  This should be fairly arbitrary, I should be able to go from one frequency to a second to a third, etc, with no expectation that I'm going back to the first one, so it's not really an FSK sort of task.

So, how should I do this, and what are my restrictions?  Do I have to use the F1 and F2 register sets as a double-buffer and ping-pong between them?  Can I do it just as well using only writing to F1 and ignoring F2?  Do I need to hold the integer portion of the N divider constant to get monotonicity?  Do I have to hold DEN constant?  I assume I can't change the output dividers on the fly without a hiccup.  Should I definitely set FCAL_EN?  Definitely not set it?

I can work around most restrictions if I know they're coming, but I don't have access to a modulation-domain analyzer so it's not practical for me to just keep playing with potential settings until I figure out what does and doesn't work.

  • 0
    TI__Guru** 115140 points

    Hi Rob,

    There is restriction on the frequency deviation in FSK mode, so if your delta frequency between current and new frequency is bigger than the FSK mode can support, you actually cannot use FSK mode.

    F1F2 mode allows you to toggle between two predefined (calibrated) VCO frequencies very fast. If you have a third frequency, a calibration is needed. See datasheet section 8.1.4 for details.

    If you are doing frequency hopping, and want to have as short switching time as possible, you may need LMX2820. This device has a InstCal function, all you have to do is perform a calibration for the whole VCO coverage once. After calibration, you can change to any frequency in approx. 2.5µs.